Various conventional methods of producing electrodes for alkaline storage batteries entail the preparation of an electrode support in the form of a porous sintered plate which is subsequently impregnated with an electrochemically active material. Thus, for example, it is known to impregnate a sintered nickel plate with a cadmium or nickel salt and to hydrolyze the salt in a strongly alkaline medium, followed by washing and drying. This cycle of operations must generally be repeated several times to permit a sufficient quantity of the active material to be incorporated in the support, rendering this method lengthy, complicated and expensive. Further, it is difficult to obtain a good reproducibility of the quantities of active material which are incorporated in the electrodes by impregnation. Moreover, the impregnating method causes a relatively high corrosion of the porous nickel support, due to the acid nitrates generally employed for impregnating, whereby the mechanical strength of the support is considerably weakened with consequent reduction of the useful life of the electrodes produced in this manner. Furthermore, the use of highly porous metal supports which are obtained by, for example, free sintering of a nickel powder does not permit maximum utilization of the active mass, particularly with nickel electrodes. This disadvantageous effect is due to a bad random distribution of the medium size of the pores or interstices produced by the sintering process.
In fact, in such sintered structures there is generally present a high proportion of large pores (having a size of 100 microns or more) as well as a considerable proportion of very fine pores (having a size of less than 0.01 micron) and of closed pores. The presence of pores having a size of more than a few tens of microns results in poor electric contact between the active mass and the metal support; on the other hand, when there are too many excessively fine pores (of a size of less than 0.1 micron), most of them completely or partly closed, these pores are not properly filled with the active mass and are insufficiently utilized as it is difficult for the electrolyte to enter these pores.
It is also known to impregnate the porous support electrochemically with a hydroxide in a single operation which must take a relatively long time to permit sufficient penetration of the hydroxide, e.g. cadmium or nickel hydroxide, into the porous support. However, such electrochemical impregnation produces an irregular distribution of the active mass in the electrode, which is highly undesirable, the concentration of the active mass being much greater near the surface than in the interior of the electrode body.
To overcome the disadvantages of the impregnating methods referred to above, it has already been proposed to produce a cadmium electrode in a single operation by subjecting a powder mixture of nickel and cadmium oxide to free sintering in a neutral atmosphere at a temperature sufficient to permit sintering of the nickel at ambient pressure. However, in the range between 700.degree. and 900.degree.C, at which this sintering is carried out, the cadmium oxide tends to sublime, resulting in an appreciable loss of active mass, the amount of sublimed cadmium oxide being a function of the duration and temperature of the sintering operation. Therefore, such free sintering of the nickel of the support in the presence of cadmium oxide, although relatively simple, generally does not yield electrode capacities as high as those obtained with the conventional electrodes referred to above in which the active mass is incorporated in the electrode by impregnation of the sintered support.
It has also been proposed to produce electrodes for storage batteries merely by pressing a powdered active mass which may be mixed with a metallic support powder. To ensure efficient cohesion of the powder particles, and thus a high mechanical strength of the electrode, very high pressures on the order of 700 to 1400 kg/ cm.sup.2 are required. By this procedure, however, the powder particles are closely compacted, causing an undesirable reduction of the final porosity of the electrode and insufficient mechanical stability of the electrode in operation.
To improve the mechanical strength of the pressed electrode body thus obtained, it has also already been proposed to subject it to sintering. However, this greatly limits the number of available combinations of active mass and support material. In fact, the sintering temperature of the support material is generally not very different from the decomposition temperature of the active mass of the electrode and therefore it is difficult to obtain satisfactory sintering without decomposition and/or volatilization of the active mass.
It has also been proposed to agglomerate particles of active material by sintering and to bind the sintered mass thus obtained to a support material by the application of pressure and heat. However, it is obvious that, because of the often very different nature of the active mass and that of the support material, the number of available combinations of active mass and support material to be sintered according to this method is likewise very limited. Moreover, it is difficult to obtain by this method a high mechanical strength and conductivity simultaneously with a satisfactory capacity.
Further, a sintered electrode is known which is made of silver grains as the active mass and nickel grains as the support material. Sintering of this electrode is effected in a reducing atmosphere at a temperature between 700.degree. and 900.degree.C; the silver grains may be obtained by decomposition of silver oxide particles during the sintering operation. To facilitate bonding between the grains it has also been proposed to subject the starting mixture during sintering to light compression with a pressure on the order of one kilogram per square centimeter. However, this method is essentially suitable only for producing an electrode having an active mass of silver, and its range of sintering temperatures cannot be utilized for producing other types of electrodes in which the active mass would decompose at such temperatures. pg,6
In addition, a cobalt electrode has been proposed in which cobalt is used for the active mass as well as for the support material. For producing such an electrode a method is used which consists of hot pressing a powder mixture of metallic cobalt and filling material which is subsequently removed to form pores in a sintered cobalt structure. This hot pressing is designed to affect sintering at a temperature such that the filling material will not chemically attack the cobalt powder and is not itself chemically attacked not undergoes a change of state during sintering. Thus, this method affords certain advantages but only in the production of cobalt electrodes. This type of negative electrode has, however, some limitations due to a progressive dissolution of Co(OH).sub.2 in an alkaline medium and therefore does not strictly meet the requirements of a long electrode life with a predetermined capacity.